purrCat

This is a novelty PCB Art of a purring cat.

Feel free to use this project or parts of it at will, but please give credit when doing so.

While this is an open source project, I am selling kits and assembled units for 10 and 15 USD respectivly.
Please consider supporting us by ordering here.

Art

The art was made by Taly Reznik - @trollworks
Go check out her awesome work!

Hardware Description

The project is based on TI's msp430fr2111 MCU. The current software could fit in (and easily compiled for) msp430fr2110 and could probably be further optimized to fit msp430fr2100, but going with the 2111 model leaves a lot of room for future expansion with just a small price difference.

Purring is achived with a round radial vibration motor, which is low-side controlled with an nmos transistor.
In addition, the eyes light up using reverse-mounted LEDs (might be worthwhile to change it to a side view LED in the future).

Petting is detected using capacitive touch.
There are 4 touch pads on the PCB, but only 1 is in use in the current FW.

Some FW details

Picky Cat!

I wanted to make the cat as realistic as possible, so it only likes petting between 0.5 and 2Hz.
(Since I'm not doing swipe detection, it would also work with tapping. I might change it in a future version)
This is achived using a 2s sample buffer with 125ms slots. if the pad is touched for more than 3 slots in a row, the buffer zeroes.
Furthermore, purring only starts after at least 1s of petting.

Realistic Purring

I have programmed a simple PWM function to make the vibration feel like real purring.
It has dynamic change to the duty cycle, this creates the subtle ramp-up and ramp-down typical of a cat's purring.
I tried a sinusoidal PWM function, but it didn't have the right feel to it.

Programming and flashing

The MCU is programmed using Code Composer Studio IDE.
For software flashing I use a TI dev board LAUNCHPAD MSP-EXP430FR2433, which is the lowest cost programmer option available, though I assume you could make a flashing device out of an arduino.
I left 3x2mm pitch pads for programming right near the MCU.
They are arranged (from top to bottom) GND-SBWTCK-SBWTDIO, an additional connection to VCC is required and could be obtained from the battery clip.

Power Consumption

The FW used to feature a distinct power saving mode, but changing the wait timer to RTC with VLO clock source randered that mode as unnedded.
It is still available in code using a #define flag.

In the current configuration, sampling consumes about 2 micro Ampere - around 60uW at nominal 3V.
When Purring, i.e. when the motor and the LEDs are enabled, the current consumption is about 22mA - 66mW at nonminal 3V.

The final design uses a cr2032 lithium cell which are rated at about 240mAh.
While theoretical battery life should have been roughly 8 hours of purring or about 6 years of idle,
The 240mAh rating is given at much lower current draw than that of the active purring.
At 20mA, most cr2032 have a capacity of a mere 10-15mAh, giving only about 30 minutes of active purring.

I don't have a good explantion for it, but idle time also seems to substencialy lower than the theoretical - getting to around one week.

Note that this is not linear and even a small reduction in current draw while purring would result in longer operation time.
Sadly, I was'nt able to find a way to reduce to consumption enough.

LiPo mod

Giving the poor battery life, I designed a small PCB holding an LDO and a charge connector for a LiPo, to be soldered instead of the battery clip. The thought of using a LiPo crossed my mind several times, but I deemed it unvaiabel because of cost and required PCB space for a charge controller. Finally, I had the realization to use a generic LiPo charger externaly (which could be obtained from AliExpress for very cheap) instead of making it onboard. I did changed the current select resistor on the charger such that the max charge current will be 100 mA - suitable for the tiny LiPo I use.

I decided to use a 501015 60mAh battery. Because these are rated at 1C, we get a runtime much closer to the theoretical value - about 1.5 hours of purring! I have used a sub-optimal LDO with 30 uA quiescent current, which becomes the dominant current draw during idle. I calculated arround 40 days of idle time, but have not ran this long time test yet.

Parts List

• MCU: msp430fr2111
• LED: 2x 0805 LED (reverse mounted, color of your choice)
• Capacitors: 2x0805 10uF MLCC (1 for the MCU and 1 for the Motor), 1x0603 100nF MLCC (for the MCU)
• Resistors: 3x0603 1k Ohm (current limiting for the transistor and the LEDs), 1x0603 100k Ohm (pulldown for the transistor, could be omitted as the driver pin is push-pull)
• Diode: 1x0603 schottky diode (I used Kyocera AVX SD0603S040S0R2)
• Transistor: 1xSOT3 NMOS Transistor (I used Nexperia NXV55UNR)
• Battery clip: 1x Keystone 3034 (cr2032 battery holder), note that not all battery holders can fit in the PCBs footprint)
• Vibration motor: 1x 10mm round radial vibration motor(or smaller), note that there are no pads for soldering it. one lead should be connected to vcc(I used the cap for it) and the other to the transistor drain pin (I made a bigger pad to accomodate this).